I have a data structure that has been gathered as follows, where each
cell contains the subject ID if A is true, and similarly for B.
Therefore, for subject 5, I have A=yes and B=yes, whereas for subjects 6
and 11, only B=yes.

A B
5 5
6 11

I'd like to transform the data structure such that each row would have
ID, and indicator variables for A and B.

id A B
5 1 1
6 0 1
11 0 1

I've tried an approach like this, where I create indicator variables but
A and B, but am not sure how to "expand" from the data structure where
different ID's reside in the same rows.

. gen id1=a
. gen id2=b
. gen aa=1 if a!=.
. gen bb=1 if b!=.

It also looks like -reshape- is not suitable because different ID's
reside in the same rows.